.exr file quality options for seperate renders?
Yes, another EXR would be the best option. Any other file type will have a hard time matching the quality and compression method. And it's not as if the EXR will be "bloated" with lots of passes without data. It just won't store data that you don't give it.

It'd also be more convenient to use the same types for the same things. If you're going to be pulling in passes for compositing, it makes sense to use the same type of file for that same process, even though it might contain different passes.